A WLED (White Light Emitting Diode, white light emitting diode) has advantages of a small size, pure light color, high light emitting efficiency, long service life, and the like, and is extensively applied in technical fields of display screen backlight, lighting, and the like, and in particular, is applied in mobile devices such as a smartphone and a tablet computer. Compared with other backlight technologies, it may significantly reduce a volume and weight of a device, and prolong a discharge time of a battery.
When working, the WLED requires a WLED driver and an input power source, where the WLED driver is integrated with a chip, and connected externally to a few peripheral components. In an actual application, to ensure consistency of luminance of multiple WLEDs while taking simplification of a WLED driver circuit and reduction of power consumption into account, generally, the multiple WLEDs are made into a WLED string or array. In terms of process and circuit feasibility, generally, a quantity of WLEDs that can be connected in series in one WLED string or array is approximately 11. For a device having a screen size of 4 to 6 inches, one WLED string including 11 WLEDs can meet an application requirement of the device. However, for a device having a screen size of more than 6 inches, such as a large-screen smartphone, a tablet computer, or a notebook computer, one WLED string cannot meet an application requirement of the device. Based on this, the prior art further proposes a WLED driver integrating multiple WLED string channels, where each channel corresponds to one WLED string. Therefore, when there are more channels, more WLED strings can be driven simultaneously.
As shown in FIG. 1, which shows a schematic structural diagram of a WLED driver in the prior art, the WLED driver includes a DC-DC CONVERTER (boost converter), a control circuit, and two precise programmable-controlled CSs (Current Sink, current sink 1, where the DC-DC CONVERTER is configured to regulate an output voltage VOUT to an appropriate value according to a quantity of WLEDs mounted on any channel, the first current sink CS1 is connected to the first channel IFB1, the second current sink CS2 is connected to the second channel IFB2, and each CS is configured to determine a current intensity of a WLED string corresponding to a channel to which the CS is connected. In an ideal case, the current determined by the CS and a reference voltage VREF are in a preset proportional relationship.
CS precision of each channel is a key to ensuring consistency of luminance of WLED strings between channels. Generally, a CS implementation manner is to connect a high-gain operational amplifier and a power stage to form a unit negative feedback loop. However, in an actual application process, a mismatch between an input offset voltage of the operational amplifier and a feedback resistance of the power stage may cause a current difference between different channels in the WLED driver. For a present integrated circuit manufacturing process, it is easier to implement a good match of feedback resistances, but the input offset voltage of the operational amplifier has relatively great impact on a current mismatch between channels in the WLED driver. Therefore, in the prior art, to achieve consistency of luminance of WLED strings between multiple channels, a key lies in elimination of the channel current mismatch caused by the input offset voltage of the operational amplifier.
Still using FIG. 1 as an example, in the prior art, to reduce the current mismatch between the two channels IFB1 and IFB2, it is necessary to introduce an error amplifier (EA, error amplifier) with a low input offset voltage. For a CMOS (Complementary Metal Oxide Semiconductor, complementary metal oxide semiconductor) process, the input offset voltage of the error amplifier is approximately several millivolts to tens of millivolts. A method for reducing the input offset voltage of the error amplifier is to make sizes of components in the error amplifier greater. However, this causes a die size of a chip to increase, which is disadvantageous for cost control, and cannot completely eliminate the current mismatch between channels. In addition, a trimming circuit may be added to an error amplifier circuit in the prior art to reduce the input offset voltage of the error amplifier. However, this method requires that trimming should be performed on each error amplifier separately, which increases the circuit complexity and cost.
Therefore, the prior art urgently requires a method for implementing an exact current match between multiple channels integrated in a WLED driver under a prerequisite that the input offset voltage of the error amplifier is not eliminated, to ensure consistency of luminance of WLEDs between multiple channels.